Add ext full backup (#1281)

* initial

Signed-off-by: hillium <yujuncen@pingcap.com>

* added hint of prefix

Signed-off-by: hillium <yujuncen@pingcap.com>

* added force flush

Signed-off-by: hillium <yujuncen@pingcap.com>

* make result repeated

Signed-off-by: hillium <yujuncen@pingcap.com>

* make full backup a reference

Signed-off-by: hillium <yujuncen@pingcap.com>

* Revert "added force flush"

This reverts commit a51d008.

Signed-off-by: hillium <yujuncen@pingcap.com>

* rollback irraltive changes

Signed-off-by: hillium <yu745514916@live.com>

* added more comments

Signed-off-by: hillium <yu745514916@live.com>

* address comments

Signed-off-by: hillium <yu745514916@live.com>

* rename extra full backup to ingested SSTs

Signed-off-by: hillium <yu745514916@live.com>

---------

Signed-off-by: hillium <yujuncen@pingcap.com>
Signed-off-by: hillium <yu745514916@live.com>